Skeletal members are formed of bone tissue and other structures such as cartilage. For various reasons, skeletal members sometimes fracture, weaken or deteriorate over time. In other instances, skeletal members may be deformed or diseased. In either case, treatment of the skeletal member usually requires some type of artificial support or stabilization to promote healing and/or correction of abnormalities.
With specific regard to treatment of the spine, plates or rods are typically attached to the portion of the spinal column being treated to provide the requisite amount of support and/or stabilization. In many cases, attachment of the plates or rods to the spine is accomplished by engaging a number of bone anchors, such as bone screws, to one or more vertebral bodies. In such applications, the bone screws are sometimes engaged to the vertebral bodies via extension through the pedicle which is mostly comprised of cancellous or porous bone tissue. When dealing with patients having soft bone tissue or with patients afflicted with a bone weakening disease (e.g., osteoporosis), conventional bone screws have a tendency to cut out or loosen as a result of insufficient bone strength.
To compensate for soft or weakened bone tissue, bone cement or another type of material is sometimes introduced adjacent the threaded portion of the bone screw to strengthen the bone. The bone cement provides a more secure anchoring arrangement to prevent the screw from cutting out or loosening. The cement material is typically introduced into the bone via passage through an axial opening extending along a length of the screw and exiting through a series of fenestration openings in communication with the axial opening and positioned at intermittent location along the length of the screw. Preferably, the bone cement should be distributed uniformly about the threaded portion of the bone screw with minimal disruption to the adjacent bone tissue.
In a prior method for treating the spine using bone screws, a uniform passage having a diameter equal to or slightly less than the screw diameter is formed through the pedicle region of the vertebral body. A bone screw is then threaded into the passage to a predetermined insertion depth, with the threads of the bone screw engaged tightly against adjacent bone tissue. Once the bone screw is properly positioned within the vertebral body, bone cement is injected through the axial opening in the bone screw and introduced into the bone by way of a number of the fenestration openings. Notably, this method of screw insertion and anchoring typically results in an uneven distribution of bone cement around the threaded portion of the bone screw. Additionally, rapid injection of the bone cement can lead to fluid pressure buildup, sometimes resulting in disruption of the cancellous bone tissue in the area adjacent the fenestration openings.
In another prior method for treating the spine using bone screws, a uniform passage having a diameter somewhat larger than the screw diameter is formed through the pedicle region of the vertebral body. In a specific application, the diameter of the passage is about 1.5 to 2.0 millimeters larger than the diameter of the bone screw. The bone screw is then inserted into the passage, and once properly positioned within the vertebral body, bone cement is introduced into the passage to fill up the void or spacing between the screw and the walls of the passage. However, this method of screw insertion and anchoring requires the formation of an oversized screw insertion passage extending through the pedicle. Notably, the formation of an oversized passage results in the removal of a relatively large amount of vertebral bone tissue, thereby tending to compromise the structural integrity of the pedicle.
Thus, there is a general need in the industry to provide improved surgical instrumentation and methods for forming a passage in bone having an enlarged cross-sectional portion than is currently available within the industry. The present invention meets this need and provides other benefits and advantages in a novel and unobvious manner.